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<title>The Basics</title>
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<h1>The Basics</h1>

<h2>Application</h2>

<p>Twisted programs usually work with
<code class="API">twisted.application.service.Application</code>.
This class usually holds all persistent configuration of
a running server -- ports to bind to, places where connections
to must be kept or attempted, periodic actions to do and almost
everything else. It is the root object in a tree of services implementing
<code class="API" base="twisted.application.service">IService</code>.</p>

<p>Other HOWTOs describe how to write custom code for Applications,
but this one describes how to use already written code (which can be
part of Twisted or from a third-party Twisted plugin developer).  The
Twisted distribution comes with an important tool to deal with
Applications, <code>twistd</code>.</p>

<p><code>Application</code>s are just Python objects, which can
be created and manipulated in the same ways as any other object.
</p>

<h2>twistd</h2><a name="twistd" />

<p>The Twisted Daemon is a program that knows how to run Applications.
This program
is <code class="shell">twistd(1)</code>.  Strictly
speaking, <code class="shell">twistd</code> is not necessary --
fetching the application, getting the <code>IService</code> component,
calling <code>startService</code>, scheduling <code>stopService</code> when
the reactor shuts down, and then calling <code>reactor.run()</code> could be
done manually. <code class="shell">twistd(1)</code>, however, supplies 
many options which are highly useful for program set up.</p>

<p><code class="shell">twistd</code> supports choosing a reactor (for more on
reactors, see <a href="choosing-reactor.xhtml">Choosing a Reactor</a>), logging
to a logfile, daemonizing and more. <code class="shell">twistd</code> supports all
Applications mentioned above -- and an additional one. Sometimes
it is convenient to write the code for building a class in straight
Python. One big source of such Python files is the <code>doc/examples</code>
directory. When a straight Python file which defines an <code>Application</code>
object called <code>application</code> is used, use the <code class="shell">-y</code>
option.</p>

<p>When <code class="shell">twistd</code> runs, it records its process id in a
<code>twistd.pid</code> file (this can be configured via a command line
switch). In order to shutdown the <code class="shell">twistd</code> process, kill that
pid (usually you would do <code class="shell">kill `cat twistd.pid`</code>).
</p>

<p>As always, the gory details are in the manual page.</p>

<h2>tap2deb</h2>

<p>
For Twisted-based server application developers who want to deploy on
Debian, Twisted supplies the <code class="shell">tap2deb</code> program. This program
wraps a Twisted Application file (of any of the supported formats -- Python,
source, xml or pickle)
in a Debian package, including correct installation and removal scripts
and <code>init.d</code> scripts. This frees the installer from manually
stopping or starting the service, and will make sure it goes properly up
on startup and down on shutdown and that it obeys the init levels.
</p>

<p>
For the more savvy Debian users, the
<code class="shell">tap2deb</code> also generates the source package, allowing her
to modify and polish things which automated software cannot detect
(such as dependencies or relationships to virtual packages). In addition,
the Twisted team itself intends to produce Debian packages for some common
services, such as web servers and an inetd replacement. Those packages
will enjoy the best of all worlds -- both the consistency which comes
from being based on the <code class="shell">tap2deb</code> and the delicate manual
tweaking of a Debian maintainer, insuring perfect integration with
Debian.
</p>

<p>
Right now, there is a beta Debian archive of a web server available at
<a href="http://twistedmatrix.com/users/moshez/apt">Moshe's archive</a>.
</p>

<h2>tap2rpm</h2>

<p><code class="shell">tap2rpm</code> is similar to <code class="shell">tap2deb</code>, except that
it generates RPMs for Redhat and other related platforms.</p>

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